Pneumatic product on demand delivery systems have been used on agricultural seeding machines to automatically direct seed from a main seed hopper to a plurality of individual planting units. Each of the individual planting units has an auxiliary seed hopper for receiving the seed, a seed meter for metering the seed from the auxiliary seed hopper and a furrow opener for forming a planting furrow into which the metered seed is deposited. A fan is used to create pressurized air that forms an air stream on which the seed is taken to the planting units. These systems automatically replenish the auxiliary hoppers as needed.
Air seeders meter seed upstream of the transporting air stream. As the meter is more remote from the application device the product is not metered onto the field in as accurate a manner as an application unit where the product meters are in close proximity to the application site.
The auxiliary seed hoppers on commercially available pneumatic seed on demand delivery systems are provided with vents for venting excess pneumatic pressure from the auxiliary hopper. In at least one commercially available system the vent is located in the sidewall of the auxiliary hopper and is provided with a screen that is fixed thereto. The screen is protected by an exterior wall that extends outward from the auxiliary hopper. The exterior wall is provided with an open bottom through which air can vent. Because of the fixed nature of this screen it is more difficult to clean and replace if damaged.